import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;

/**
 * Created with IntelliJ IDEA.
 * Description:
 * User: 86186
 * Date: 2023-10-27
 * Time: 18:10
 */
public class Test {
    public static void main1(String[] args) {
        Scanner in = new Scanner(System.in);
        // 注意 hasNext 和 hasNextLine 的区别
        while (in.hasNextInt()) { // 注意 while 处理多个 case
            int n = in.nextInt();
            int ablt = in.nextInt();
            int[] t = new int[n];
            for (int i = 0; i < n; i++) {
                t[i] = in.nextInt();
            }
            for (int i = 0; i < n; i++) {
                if (ablt >= t[i]) {
                    ablt += t[i];
                }else {
                    ablt += gcd(t[i], ablt);
                }
            }
            System.out.println(ablt);
        }
    }
    public static int gcd(int a, int b) {
        while (a % b != 0) {
            int c = a % b;
            a = b;
            b = c;
        }
        return b;
    }

    public static void main2(String[] args) {
        Scanner in = new Scanner(System.in);
        // 注意 hasNext 和 hasNextLine 的区别
        Map<Character,Integer> hash = new HashMap<>();
        String s = in.next();
        for (int i = 0; i < s.length(); i++) {
            char ch = s.charAt(i);
            hash.put(ch, hash.getOrDefault(ch,0) + 1);
        }
        for (int i = 0; i < s.length(); i++) {
            char ch = s.charAt(i);
            if (hash.get(ch) == 1) {
                System.out.println(ch);
                return;
            }
        }
        System.out.println(-1);
    }

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        // 注意 hasNext 和 hasNextLine 的区别
        String s = in.next();
        int n = s.length();
        int[] hash = new int[128];
        for (int i = 0; i < n; i++) {
            char ch = s.charAt(i);
            hash[(int)ch]++;
        }
        for (int i = 0; i < n; i++) {
            char ch = s.charAt(i);
            if (hash[(int)(ch)] == 1) {
                System.out.println(ch);
                return;
            }
        }
        System.out.println(-1);

        StringBuilder sb = new StringBuilder();
        sb.de
    }
}
